diff --git a/apps/web/src/components/chat/MessagesTimeline.logic.test.ts b/apps/web/src/components/chat/MessagesTimeline.logic.test.ts
new file mode 100644
index 0000000000..7074f46019
--- /dev/null
+++ b/apps/web/src/components/chat/MessagesTimeline.logic.test.ts
@@ -0,0 +1,135 @@
+import { describe, expect, it } from "vitest";
+import { computeMessageDurationStart } from "./MessagesTimeline.logic";
+
+describe("computeMessageDurationStart", () => {
+  it("returns message createdAt when there is no preceding user message", () => {
+    const result = computeMessageDurationStart([
+      {
+        id: "a1",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:05Z",
+        completedAt: "2026-01-01T00:00:10Z",
+      },
+    ]);
+    expect(result).toEqual(new Map([["a1", "2026-01-01T00:00:05Z"]]));
+  });
+
+  it("uses the user message createdAt for the first assistant response", () => {
+    const result = computeMessageDurationStart([
+      { id: "u1", role: "user", createdAt: "2026-01-01T00:00:00Z" },
+      {
+        id: "a1",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:30Z",
+        completedAt: "2026-01-01T00:00:30Z",
+      },
+    ]);
+
+    expect(result).toEqual(
+      new Map([
+        ["u1", "2026-01-01T00:00:00Z"],
+        ["a1", "2026-01-01T00:00:00Z"],
+      ]),
+    );
+  });
+
+  it("uses the previous assistant completedAt for subsequent assistant responses", () => {
+    const result = computeMessageDurationStart([
+      { id: "u1", role: "user", createdAt: "2026-01-01T00:00:00Z" },
+      {
+        id: "a1",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:30Z",
+        completedAt: "2026-01-01T00:00:30Z",
+      },
+      {
+        id: "a2",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:55Z",
+        completedAt: "2026-01-01T00:00:55Z",
+      },
+    ]);
+
+    expect(result).toEqual(
+      new Map([
+        ["u1", "2026-01-01T00:00:00Z"],
+        ["a1", "2026-01-01T00:00:00Z"],
+        ["a2", "2026-01-01T00:00:30Z"],
+      ]),
+    );
+  });
+
+  it("does not advance the boundary for a streaming message without completedAt", () => {
+    const result = computeMessageDurationStart([
+      { id: "u1", role: "user", createdAt: "2026-01-01T00:00:00Z" },
+      { id: "a1", role: "assistant", createdAt: "2026-01-01T00:00:30Z" },
+      {
+        id: "a2",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:55Z",
+        completedAt: "2026-01-01T00:00:55Z",
+      },
+    ]);
+
+    expect(result).toEqual(
+      new Map([
+        ["u1", "2026-01-01T00:00:00Z"],
+        ["a1", "2026-01-01T00:00:00Z"],
+        ["a2", "2026-01-01T00:00:00Z"],
+      ]),
+    );
+  });
+
+  it("resets the boundary on a new user message", () => {
+    const result = computeMessageDurationStart([
+      { id: "u1", role: "user", createdAt: "2026-01-01T00:00:00Z" },
+      {
+        id: "a1",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:30Z",
+        completedAt: "2026-01-01T00:00:30Z",
+      },
+      { id: "u2", role: "user", createdAt: "2026-01-01T00:01:00Z" },
+      {
+        id: "a2",
+        role: "assistant",
+        createdAt: "2026-01-01T00:01:20Z",
+        completedAt: "2026-01-01T00:01:20Z",
+      },
+    ]);
+
+    expect(result).toEqual(
+      new Map([
+        ["u1", "2026-01-01T00:00:00Z"],
+        ["a1", "2026-01-01T00:00:00Z"],
+        ["u2", "2026-01-01T00:01:00Z"],
+        ["a2", "2026-01-01T00:01:00Z"],
+      ]),
+    );
+  });
+
+  it("handles system messages without affecting the boundary", () => {
+    const result = computeMessageDurationStart([
+      { id: "u1", role: "user", createdAt: "2026-01-01T00:00:00Z" },
+      { id: "s1", role: "system", createdAt: "2026-01-01T00:00:01Z" },
+      {
+        id: "a1",
+        role: "assistant",
+        createdAt: "2026-01-01T00:00:30Z",
+        completedAt: "2026-01-01T00:00:30Z",
+      },
+    ]);
+
+    expect(result).toEqual(
+      new Map([
+        ["u1", "2026-01-01T00:00:00Z"],
+        ["s1", "2026-01-01T00:00:00Z"],
+        ["a1", "2026-01-01T00:00:00Z"],
+      ]),
+    );
+  });
+
+  it("returns empty map for empty input", () => {
+    expect(computeMessageDurationStart([])).toEqual(new Map());
+  });
+});
diff --git a/apps/web/src/components/chat/MessagesTimeline.logic.ts b/apps/web/src/components/chat/MessagesTimeline.logic.ts
new file mode 100644
index 0000000000..45408468ca
--- /dev/null
+++ b/apps/web/src/components/chat/MessagesTimeline.logic.ts
@@ -0,0 +1,25 @@
+export interface TimelineDurationMessage {
+  id: string;
+  role: "user" | "assistant" | "system";
+  createdAt: string;
+  completedAt?: string | undefined;
+}
+
+export function computeMessageDurationStart(
+  messages: ReadonlyArray<TimelineDurationMessage>,
+): Map<string, string> {
+  const result = new Map<string, string>();
+  let lastBoundary: string | null = null;
+
+  for (const message of messages) {
+    if (message.role === "user") {
+      lastBoundary = message.createdAt;
+    }
+    result.set(message.id, lastBoundary ?? message.createdAt);
+    if (message.role === "assistant" && message.completedAt) {
+      lastBoundary = message.completedAt;
+    }
+  }
+
+  return result;
+}
diff --git a/apps/web/src/components/chat/MessagesTimeline.tsx b/apps/web/src/components/chat/MessagesTimeline.tsx
index 3bfef9d87c..7a89e762e3 100644
--- a/apps/web/src/components/chat/MessagesTimeline.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.tsx
@@ -19,6 +19,7 @@ import { ProposedPlanCard } from "./ProposedPlanCard";
 import { ChangedFilesTree } from "./ChangedFilesTree";
 import { DiffStatLabel, hasNonZeroStat } from "./DiffStatLabel";
 import { MessageCopyButton } from "./MessageCopyButton";
+import { computeMessageDurationStart } from "./MessagesTimeline.logic";
 
 const MAX_VISIBLE_WORK_LOG_ENTRIES = 6;
 const ALWAYS_UNVIRTUALIZED_TAIL_ROWS = 8;
@@ -98,6 +99,9 @@ export const MessagesTimeline = memo(function MessagesTimeline({
 
   const rows = useMemo<TimelineRow[]>(() => {
     const nextRows: TimelineRow[] = [];
+    const durationStartByMessageId = computeMessageDurationStart(
+      timelineEntries.flatMap((entry) => (entry.kind === "message" ? [entry.message] : [])),
+    );
 
     for (let index = 0; index < timelineEntries.length; index += 1) {
       const timelineEntry = timelineEntries[index];
@@ -139,6 +143,8 @@ export const MessagesTimeline = memo(function MessagesTimeline({
         id: timelineEntry.id,
         createdAt: timelineEntry.createdAt,
         message: timelineEntry.message,
+        durationStart:
+          durationStartByMessageId.get(timelineEntry.message.id) ?? timelineEntry.message.createdAt,
         showCompletionDivider:
           timelineEntry.message.role === "assistant" &&
           completionDividerBeforeEntryId === timelineEntry.id,
@@ -507,8 +513,8 @@ export const MessagesTimeline = memo(function MessagesTimeline({
                   {formatMessageMeta(
                     row.message.createdAt,
                     row.message.streaming
-                      ? formatElapsed(row.message.createdAt, nowIso)
-                      : formatElapsed(row.message.createdAt, row.message.completedAt),
+                      ? formatElapsed(row.durationStart, nowIso)
+                      : formatElapsed(row.durationStart, row.message.completedAt),
                   )}
                 </p>
               </div>
@@ -605,6 +611,7 @@ type TimelineRow =
       id: string;
       createdAt: string;
       message: TimelineMessage;
+      durationStart: string;
       showCompletionDivider: boolean;
     }
   | {